1. Field of the Invention
The invention relates to a control device and method for a variable valve mechanism. More particularly, the invention relates to a control device and method for a variable valve mechanism that changes the operational characteristic of an intake valve in an internal combustion engine.
2. Field of the Invention
In a conventional control device for an internal combustion engine with a variable valve mechanism, Japanese Patent Application Publication No. 2001-263015 (JP 2001-263015) describes a variable valve mechanism that continuously changes the lift of an intake valve and the period in which the intake valve is held open. The term “duration” indicates the period of time that the intake valve is held open in terms of crank angle.
The above-described variable valve mechanism includes an intermediate drive mechanism provided between a camshaft and the intake valve. A support pipe oscillably supports the intermediate drive mechanism. When an intake cam contacts the input portion of the intermediate drive mechanism to drive the intermediate drive mechanism, the oscillating cam of the intermediate drive mechanism drives the intake valve via a rocker arm. A variable-lift actuator changes the lift by changing the difference between the phases of the oscillating cam and the input portion of the intermediate drive mechanism. Thus, the lift and duration of the intake valve are continuously adjusted.
In the above-described variable valve mechanism, the variable-lift actuator drives a control shaft, and the control shaft changes the difference between the phases of the oscillating cam and the input portion. As a result, it is possible to change the lift and duration of the intake valve that is moved by the rotation of the intake cam.
For example, when the engine starts, the viscosity of lubricating oil may be high due to the low temperature of the engine or motor. In this case, the variable-lift actuator may not move smoothly. Therefore, when the engine starts, it is desirable to execute a control that maintains the duration at a large value, and adjusts the intake air amount by changing the opening amount of the throttle valve. After the engine is warmed up completed, it is desirable to start another control that changes the duration.
When an operation mode is changed to decrease the duration (the period in which the intake valve is held open), the duration and the intake air amount do not change uniformly. Therefore, the variable valve mechanism needs to be controlled so that the driver does not feel discomfort.